Power semiconductor devices such as diodes, SCRs and thyristors are typically applied in, although not limited to, rectifier and AC switch circuits to regulate the flow of power from an AC source to an electrical load. Such an electrical load may be, for example, an AC or DC motor for a liquid chiller in an air conditioning system. The semiconductor devices are rated and applied to conduct significant amounts of current and, in the process, generate corresponding significant amounts of heat. The generated heat results in an increase in the junction temperature of the semiconductor device. The devices are normally rated for current carrying capacity as a function of junction cooling capability, with the rated amount of current increasing with increasing cooling capacity. Exceeding the rated current for a particular junction temperature degrades the device and can result in device failure. It is desirable, in order to achieve maximum utilization of power semiconductor devices, to provide device cooling to enable greater amounts of current to be conducted by the device without exceeding the device junction temperature rating.
Power semiconductor devices are manufactured in at least two types according to the manner in which the electrical connections of the device are made accessiable. Those two types are referred to as the stud type and disc type. As explained below, the preferred embodiment of the present invention is directed to the mounting and cooling of disc type semiconductor devices. The principles of the invention, however, may be applicable to stud type devices.
As is well known in the art, such disc type devices consist of the semiconductor device, e.g., diode, transistor or thyristor, positioned within a disc shaped container having opposing metallic pole faces that are spaced from one another by an annular insulating skirt. Upon mounting the disc device for circuit operation, it is necessary to apply a compressive force to the opposing pole faces to bring them into contact with the respective proximate anode or cathode of the semiconductor device contained within the disc shaped container. The disc device mounting configurations known in the art provide some cooling for the device and also provide a mechanism for applying the requisite compressive mounting force to the disc faces.
One mounting configuration known in the art for mounting a disc type device is illustrated in FIG. 1. The mounting configuration includes a double-sided, fin cooled apparatus 100 in which a disc device 102 is compressed between two air-cooled, metallic finned heat sinks 104 and 106. Hardware for clamping the two heat sinks together to achieve the requisite compressive mounting force on disc device 102 is not shown. Hardware for accessing the pole faces of device 102 for electrical connection thereto is also not shown in the figure.
It is also known to mount disc type devices in pairs with the two disc devices mounted in an electrically parallel configuration, the anode and cathode of the respective devices being connected together. Such parallel mounting is provided because of the utility of that arrangement in various circuit configurations. One parallel mounting configuration known in the art is illustrated in FIG. 2. The illustrated disc device mounting and cooling apparatus 200 includes disc type devices 202 and 20 mounted between first and second bus terminal bars 206 and 208. Devices 202 and 204 can thus be mounted between terminal bars 206 and 208 in the electrically parallel configuration described above. Terminal bar 206 is electrically insulated from a water-cooled plate 210 by a sheet of thermally conductive, electrically insulating material 212. Apparatus 200 may alternatively be configured to substitute an air-cooled finned heat sink (not shown in FIG. 2) in place of plate 210. Apparatus 200 further includes a pressure plate 214 for applying pressure to terminal bar 208 via ball bearings 216 and 218 and metallic discs 220 and 222, the combination of bearings and metallic discs serving to evenly distribute the force applied to disc devices 202 and 204. A through bolt 224 is affixed to plate 210, the bolt extending through and being insulated from (by means not shown) terminal bars 206 and 208. Bolt 224 extends through pressure plate 214. By tightening a nut 226, over a washer 228, onto the pressure plate, a suitable compressive force is applied to devices 202 and 204. A sheet of electrically insulating material 230 is provided to electrically isolate discs 220 and 222 and pressure plate 214 from terminal bar 208. The means for cooling each disc device 202,204 is through the pole face thereof in contact with terminal bar 206, the heat being conducted from the terminal bar, through electrically insulating sheet 212, to plate 210. Apparatus generally configured as apparatus 200 is commercially available as open power modules from Powerex, Inc., of Youngwood, Pa. Apparatus 200 only affords cooling of one side of each semiconductor disc device and, therefore, may have a limited current carrying capacity or need to be constructed to have a sufficiently large size to meet a current carrying requirement.
Another known manner of mounting and cooling semiconductor devices is illustrated in FIG. 3. The disc mounting and cooling apparatus 300 includes two semiconductor disc devices 302 and 304 which are each provided with cooling on both of their respective pole faces. Apparatus 300 includes cylindrical metallic masses 306, 308 and 310 between which are interposed the disc devices. The cylindrical masses are in thermal and electrical conductive contact with the disc devices. The disc devices and metallic masses rest in a cylindrical shaped depression of a metallic base 312, a layer of insulating material (not shown) being provided to electrically insulate the devices and masses from the base. Apparatus 300 further includes a water-cooled mounting plate 314, in thermal conductive contact with base 312, for cooling the disc devices. Arrows 316 and 318 are provided to indicate the circulation of cooling water through plate 314. Apparatus 300 also includes a cover 320, partially shown in FIG. 3, which fits over the devices and masses and onto base 312. Since each mass can be at an electrical potential, cover 320 is electrically isolated from them (by means not shown). Further, the cover includes three openings through which pass insulating sleeves 322, 324 and 326, electrical leads corresponding to the disc device pole faces being brought out of device 300 via these sleeves. Apparatus 300 further includes means such as springs mounted at opposing ends within cover 320 to apply a compressive mounting force F.sub.c to the disc devices via the cylindrical masses. The heat generated by each disc device is conducted through each pole face to the proximate cylindrical mass and from the cylindrical mass to the metallic base 312 and the water-cooled plate 314. Apparatus generally configured as illustrated in FIG. 3 is commercially available as closed power modules manufactured by Powerex, Inc.
The inventors of the present invention have found that none of the above mounting systems have provided optimum results, when power semiconductor devices are applied in power supply circuits for large motors. By means of example only, a plurality of semiconductor devices are used in starting circuits for motors in substantial air conditioning and heat pump systems. The motors typically have horsepower ratings within the range of 100 to 1000 hp, and during start up and operation the semiconductor devices generate significant amounts of heat.
The typical duty cycle to which the semiconductor devices are subjected in such circuits is illustrated in FIG. 4 which shows a plot 400 of semiconductor device power dissipation P.sub.D as a function of time. The duty cycle is characterized by an initial transient period of operation 402 during motor starting, followed by a steady-state period of operation 404 during motor running. As an example, a starting current I.sub.start of the motor may have a magnitude that is approximately 3 times a motor running current I.sub.run, i.e., I.sub.start .congruent.3.times.I.sub.run, and the transient period may last in excess of 30 seconds. In such a case, the semiconductor device power dissipation during starting, P.sub.start, will be approximately four times the device power dissipation during running, i.e., P.sub.run. The mounting and cooling configuration selected for the semiconductor devices applied in the motor power supply circuit must be capable of carrying the starting current for the duration of the transient starting period and the running current on a continuous basis without exceeding a maximum allowable junction temperature of the semiconductor devices.
In order to carry significant levels of starting current, e.g., in excess of 3000 amps, disc device mounting and cooling apparatus of the type illustrated in FIG. 1 must be physically quite large so that substantial physical space must be allocated therefor. This size limitation has not been acceptable in commercial applications.
The mounting system illustrated in FIG. 2 also has significant limits. By means of example, FIG. 5 illustrates a plot 500 of semiconductor device junction temperature T.sub.j as a function of time, with the motor starting and running period being delineated in the figure. A maximum junction temperature of 125.degree. C. is assumed, although semiconductor devices with other maximum junction temperatures are available. The junction temperature characteristic curves shown in FIG. 5 all correspond to the same exemplary duty cycle. That duty cycle consists of a motor starting current of 1400 amps for 40 seconds and a steady state running current of 510 amps. Curve 502 (solid line) corresponds to the junction temperature of semiconductor devices mounted as illustrated in FIG. 2 with respect to apparatus 200. As can be seen, the cooling of only one side of each semiconductor device in apparatus 200 results in an unacceptable cooling capability during motor starting as compared to the motor running period. The user is, therefore, left with the alternatives of either derating the starting capability or using a mounting system 200 with semiconductor devices having a larger rating so that the maximum junction temperature is not exceeded. Choosing the former alternative results in limited starting capabilities. The latter alternative leads to greater expense and physical size of the apparatus. Further, with respect to the latter alternative, while the maximum junction temperature will not be exceeded, the junction temperature during the steady state operation period will be substantially below the maximum junction temperature, e.g., by 50% Such a substantial temperature margin during steady-state operation represents an inefficient use of the semiconductor devices, at least from the view of costs.
The mounting system shown in FIG. 3 also has been less than fully acceptable. Curve 504 (alternating long and short dashes) corresponds to the junction temperature of semiconductor devices mounted such as in apparatus 300 (FIG. 3) and applied to the motor power supply circuit. As can be seen, that apparatus is capable of providing adequate device cooling during the transient operation period such that the maximum junction temperature is not exceeded. However, the junction temperature experienced during the steady-state operation period, e.g., 60.degree. C., is substantially below the maximum junction temperature, i.e., by more than 50%. As indicated above, such a substantial temperature margin during steady-state operation represents an inefficient use of the semiconductor devices. Further, the user bears the cost of obtaining the cooling capacity required to accommodate the transient operation, while such cooling capacity is greatly underutilized during the steady-state operation period.